Brushless servo motors have been widely used in the industry, especially in the area of factory automation. The brushless servo motors that are most commonly used are those with a continuous power output range of 100 W to 1 KW. These motors are produced in high volumes, and typically with 3 phases which work with standard servo amplifiers that are commonly available. Hence, brushless servo motors are very economical. However, for many applications, the torque produced by such servo motors is not sufficient, while the speed required is much less than the nominal speed of these motors. Hence, many transmission devices such as gears and belts are used with servo motors to increase the torque output, as well as reduce the speed of rotation. The transmission devices cause inaccuracies due to backlash and are susceptible to wear and tear. Moreover, the transmission devices add to the moment of inertia of the motor, thereby reducing its dynamic performance.
Therefore, in recent years, direct drive rotary servo motors have become more common, and are replacing more and more conventional brushless servo motors in applications. The direct drive servo motor has a bigger diameter and produces a larger torque, which enables it to drive the load directly without any transmission device. Since the torque required from a direct drive rotary motor has to be large enough to drive a load directly, various methods have been used to increase torque output.
FIG. 1 (prior art) shows a conventional direct drive motor. The motor comprises a stator 11, which is typically made from stacked laminated silicon steel sheets, with coils 12 surrounding the teeth of the stator 11. Rare earth magnets 13 are placed on the outer diameter of the rotor 14. The rotor 14 can be made from a solid piece of magnetic steel which has very good permeability, or it can be made from stacked laminated silicon steel steels like the stator 11. The magnetic flux circuit 15 shows that the flux flows through the magnets with different polarity, and the flux is closed on the stator coil back iron, as well as the rotor magnet back iron.
U.S. Design Pat. No. US D565,617 S illustrates a direct drive motor using the conventional design described above. Instead of an inner rotor, another U.S. Pat. No. 4,853,567 describes a direct drive motor with an inner stator and outer rotor. Since the moment of inertia of a rotating object is proportional to the square of the distance between the centre of rotation and the centre of mass of the object, with an outer rotor, the rotor inertia becomes significantly larger.
To increase the torque of a direct drive motor, one method used is to add cooling to the motor. US Patent Publication No. 2008/0164773 A1 describes a cooling method for the stator of a direct drive motor. While cooling can increase the continuous torque of a motor, the additional facilities required include a circulating coolant and a heat exchanger to remove heat. The maintenance of these facilities adds to the costs of using such a motor.
Another way to increase the torque of a motor is described in US patent application US 2011/0101810 A1, where two rotors are connected in series to increase the torque output. A disadvantage of this design is that the motor becomes longer in the axial direction, which is undesirable for applications where there is a height constraint.
What is desirable is a motor with a low rotor inertia, but with higher torque output.